The present invention relates to a display device and a method of driving the display device, and particularly to a display device including small-sized two-color ball type electrophoretic particles each of which is composed of hemispherical portions different in color from each other for performing various kinds of display by making use of the electrophoretic particles, and a method of driving the display device.
In recent years, CRTs and liquid crystal displays have been mainly used as display devices. However, light emission type displays such as CRTs are not suitable for reading of documents, etc., because they cause fatigue of eyes of a viewer. Meanwhile, as for liquid crystal displays, a type adopting a backlight tends to cause fatigue of eyes like CRTs, and a type not adopting a backlight is disadvantageous in that the contrast is poor, which also tends to cause strong fatigue to eyes of a viewer if the viewer has a look at the screen for a long period of time. Also, these displays generally have no memory capabilities, to give rise to a disadvantage that an image disappears when a power supply is cut off.
In view of the foregoing, displays used for portable information equipment expected to be widespread for the future, for example, PDAs, note type personal computers and electronic book players, are required to be reduced in power consumption and to have a memory capability of images.
As the display capable of satisfying the above-described requirements to some extent, there have been known an electrophoretic display device and a two-color ball display device.
The electrophoretic display device is configured to make use of a principle in which electrophoretic particles composed of charged fine particles migrate toward an electrode having a polarity reversed to that of electric charges of the electrophoretic particles due to the effect of an electric field.
The electrophoretic display device has a configuration, for example, shown in FIG. 18, in which a transparent substrate 102 provided with transparent electrodes 101 made from ITO or the like is opposed to a substrate 104 provided with electrodes 103 not requiring transparency with a specific gap kept therebetween, and the gap formed between the transparent substrate 102 and the substrate 104 is filled with an electrophoretic particles 105 composed of, for example, white charged particles, and a dispersion medium 106 in which the electrophoretic particles 105 are dispersed. The electrophoretic particles 105 are made from, for example, a white pigment, and the dispersion medium 106 is colored into, for example, black.
In the above electrophoretic display device, as shown by a portion A in FIG. 18, if the electrophoretic particles 105 are negatively charged, when a plus voltage is applied to the transparent electrode 101 and a minus voltage is applied to the other electrode 103, the electrophoretic particles 105 migrate to the plus side electrode, that is, to the transparent electrode 101 by coulomb forces and adhere on the transparent electrode 101. When a viewer turns his eyes upon the portion A of electrophoretic display device from an eye position E shown in FIG. 18, he perceives through the transparent electrode 101 and the transparent substrate 102 that the portion, on which the white charged particles (electrophoretic particles 105) adhere, of the transparent electrode 101 is white-colored.
When the polarity of the applied voltage is reversed, as shown by a portion B in FIG. 18, the white charged particles (electrophoretic particles 105) migrate to the back side electrode, that is, to the electrode 103 and adhere thereon, and accordingly, they are hidden by the black dispersion medium 106. When a viewer turns his eyes upon the portion B from the eye position E in FIG. 18, he perceives that the portion B is black-colored. In addition, according to this electrophoretic display device, if the white charged particles (electrophoretic particles 105) adhere on the electrode 101 (or 103), they stand still for a short while after cutoff of the applied voltage.
In the above-described electrophoretic display device, either the color of the electrophoretic particles 105 or the color of the dispersion medium 106 is displayed. If the color of the electrophoretic particles 105 is white and the color of the dispersion medium 106 is black as described above, since the black dispersion medium 106 remains in gaps among the white particles 105 collected by migration, the display of clear white cannot be realized. If the color of the electrophoretic particles 105 is black and the color of the dispersion medium 106 is white, it is possible to realize the display of clear white by increasing the density of the white color of the dispersion medium 106;, however, in this case, upon black display, a dense white dispersion medium 106 remains in gaps among the black particles 105, so that the display of true black cannot be realized. As a result, it is difficult to obtain a high contrast ratio.
The two-color ball display device includes a plurality of two-color balls each typically having a white hemispherical portion and a colored half, for example blackened hemispherical portion, which are different from each other in terms of zeta-potential. Such a display device is operated on the basis of a principle in which a large number of the above two-color balls are rotated by the effect of an electric field.
The two-color display device has a configuration, for example, shown in FIG. 19 in which a transparent substrate 111 provided with transparent electrodes 110 made from ITO or the like is opposed to a substrate 113 provided with electrodes 112 not requiring transparency with a specific gap kept therebetween, and the above gap is filled with a large number of two-color balls 114 and a liquid 115 rotatably surrounding the two-color balls 114. In the example show n in FIG. 19, white hemispherical portions 114a of the two-color balls 114 are positively charged, and colored, for example, blackened hemispherical portions 114b thereof are negatively charged.
In this two-color ball display device , when a voltage (electric field) is applied between the electrodes 110 and 112 provided on the inner surfaces of the substrates 111 and 113 holding the two-color balls 114 as display media and the liquid 115 therebetween, each two-color ball 114 rotates on the basis of a relationship between the polarity thereof and correspons to the polarity of the voltage applied to the transparent electrode 110 or electrode 112 in such a manner that the colored hemispherical portion 114b is directed to the transparent substrate 111 side as shown by a portion A in FIG. 19 or directed to the substrate 113 side as shown by a portion B in FIG. 19. Accordingly, the white or black display can be realized by adjusting the rotation of the two-color balls 114. In the two-color ball display device, even if the supply of the voltage is cut off, the display state remains for a short while.
Even in the two-color ball display device shown in FIG. 19, however, it is difficult to obtain a high contrast ratio. This is because, the contrast between the colors of the two hemispherical portions of the two-color ball is high;, however, since a gap required for the liquid to enter between the two-color balls must be provided for rotating the two-color balls, the two-color balls cannot be closely packed.
To solve the above disadvantage, there has been proposed a display device shown in FIG. 20 in which two-color balls are arranged in multiple levels (multiple layers) for covering gaps among the two-color balls in the first layer with the two-color balls in the second layer. In such a display device, however, the white color in the second layer, which is affected by the black color in the first layer, becomes darker than the white layer in the first layer. As a result, according to this display device, it fails to obtain a sufficient effect to realize the display of clear white. Further, in this display device, since a distance between electrodes 110 and 112 becomes longer, there occurs a problem that a high voltage is required to be applied for driving the two-color balls.
In each of the two-color ball display devices shown in FIG. 19 and 20, since the display switching is performed by allowing the two-color balls to rotate and migrate, there occurs an inconvenience that it takes a time to perform the display switching.
Further, in the above electrophoretic display device and the two-color ball display device, only two colors can be displayed. To realize multi-color display of the electrophoretic display device, there may be considered a method in which cells filled with colored dispersion media of different colors, for example, the three primary colors of red, blue and green or cyan, magenta and yellow are regularly arranged, and white electrophoretic particles are dispersed in each of the colored dispersion media of the cells, wherein the white electrophoretic particles are allowed to migrate for each cell.
In this method, however, since the cells filled with the colored dispersion media of three colors are regularly arranged and must be driven through electrodes attached to respective cells, there occurs a problem that the production works become very complicated. Also, in this method, since the black display is realized by making visible the dispersion media of the three primary colors of red, blue and green or cyan, magenta and yellow, it is impossible to realize the display of dark black.
If the electrophoretic particles is colored into black for obtaining the true black display and each dispersion medium is colored into red, blue, or the like, the black display can be obtained when the electrophoretic particles are located on the transparent electrode side, however, when the particles migrate to the opposed electrode side, since light having passed through the colored dispersion medium is absorbed by the black electrophoretic particles, the visible color through the dispersion medium becomes darker than the actual color of the dispersion medium. Accordingly, the multi-color electrophoretic display device using the black electrophoretic particles can realize only the black display.
In the case of using a black dispersion medium and electrophoretic particles of the three primary colors of red, blue and green or cyan, magenta and yellow, the display of white, black, and other colors can be obtained;, however, even in this case, since the white display is realized by making visible the colors of the colored dispersion media of the three primary colors of red, blue and green or cyan, magenta and yellow, it is impossible to realize the display of clear white.
Similarly, to realize the multi-color display of the two-color ball display device, there may be considered a method in which balls of three kinds of different color, for example, a ball having white and magenta hemispherical portions, a ball having white and cyan hemispherical portions, and a ball having white and yellow hemispherical portions, are regularly arranged, and these balls are individually driven. Even in this method, however, since the different two-color balls must be regularly arranged and must be driven by electrodes attached to respective balls, there occurs a problem that the production works become very complicated. Further, even in this method, it is impossible to realize the display of true black and clear white.
By using two-color balls each being composed of a white hemispherical portion and a colored hemispherical portion colored into each of the three primary colors of red, blue and green or cyan, magenta and yellow, it is possible to realize the display of clear white;, however, since the black display is obtained by making visible the hemispherical portions of the three primary colors of red, blue and green or cyan, magenta and yellow, it is impossible to realize the display of true black. On the other hand, by using two-color balls each being composed of a black hemispherical portion and a colored hemispherical portion colored into each of the above three primary colors, it is possible to realize the display of true black, however, since the white display is obtained by making visible the hemispherical portions of the three primary colors, it is impossible to realize the display of clear white.
An object of the present invention is to provide a display device capable of realizing the display of clear white and true black, enhancing the contrast ratio, realizing high speed display switching, and performing multi-color display, and a method of driving the display device.
To achieve the above object, according to a first aspect of the present invention, there is provided a display device including: display media each of which has a dispersion medium and a large number of electrophoretic particles dispersed in the dispersion medium, the electrophoretic particles being driven by applying an electric field to each of the display media, to thereby perform a desired display operation; wherein the electrophoretic particles comprise small-sized two-color ball type electrophoretic particles (hereinafter, referred to as xe2x80x9ctwo-color ball type particlesxe2x80x9d) each of which is composed of a pair of hemispherical portions different from each other in terms of color or reflectance and charging characteristic; and the dispersion medium comprises a colorless/transparent dispersion medium.
With this configuration, since the electrophoretic particles dispersed in the dispersion medium is composed of the small-sized two-color ball type particles each of which is composed of a pair of hemispherical portions different from each other in terms of color or reflectance and charging characteristic, when the two-color ball type particles migrate to the display screen side by applying an electric field to the display medium, they are closely collected on the display screen side in such a manner that the hemispherical portions thereof on the display screen side are viewed to be closely packed, showing the color of only one side of the hemispherical portion. Also, since the dispersion medium is colorless and transparent, even if being present in gaps among the two-color ball type particles, it does not exert adverse effect on the display color. As a result, the color of the hemispherical portions on the display screen side are displayed without effect of the dispersion medium, and accordingly, if one of the pair of hemispherical portions of each two-color ball type particle is colored into white and the other is colored into black, it is possible to realize the display of clear white and true black.
The display device according to the first aspect of the present invention may include a control means for controlling a time required for applying an electric field to each of the display media in such a manner that the electric field is applied to the display medium for only a time required for the two-color ball type electrophoretic particles present on the display screen side of the display medium or the side opposed to the display screen side of the display medium to be reversed, but to migrate before reaching to the opposed side or to the display screen side. With this configuration, since the display of the color of one of the pair of hemispherical portions of each two-color ball type particle can be switched to the color of the other by applying an electric field only for a time required for the two-color ball type particle to be reversed, it is possible to shorten the display switching time.
Each of the media may be formed by filling a micro-capsule with the dispersion medium and the two-color ball type electrophoretic particles. With this configuration, it is possible to prevent localization of the two-color ball type particles and to simplify the production of the display device.
To achieve the above object, according to a second aspect of the present invention, there is provided a display device including: display media each of which has a dispersion medium and a large number of electrophoretic particles dispersed in the dispersion medium, the electrophoretic particles being driven by applying an electric field to each of the display media, to thereby perform a desired display operation; wherein the electrophoretic particles comprise small-sized two-color ball type electrophoretic particles each of which is composed of a pair of hemispherical portions different from each other in terms of color or reflectance and charging characteristic; and the dispersion medium comprises a colored dispersion medium.
With this configuration, since the electrophoretic particles are composed of the small-sized two-color ball type particles each of which is composed of a pair of hemispherical portions different from each other in terms of color or reflectance and charging characteristic, when each two-color ball type particle migrates to the display screen side, the color of one of the pair of the hemispherical portions of the two-color ball type particle is displayed. Accordingly, if one of the pair of hemispherical portions of the two-color ball type particle is colored into white and the other is colored into black, it is possible to realize the display of clear white and true black. Further, since the dispersion medium is composed of the colored dispersion medium, if one of the pair of hemispherical portions of each two-color ball type particle is colored into white which reflects light, when the two-color ball type particle migrates to the side opposed to the display screen side in such a manner that the white hemispherical portion of the two-color ball type particle is directed to the display screen side, the color of the colored dispersion medium can be displayed. Accordingly, it is possible to realize the display of a plurality of different colors.
The display device according to the second aspect of the present invention may include a control means for controlling a time required for applying an electric field to each of the display media in such a manner that the electric field is applied to the display medium only for a time required for the two-color ball type electrophoretic particles present on the display screen side of the display medium or the side opposed to the display screen side of the display medium to be reversed, but to migrate before reaching to the opposed side or to the display screen side. With this configuration, like the display device according to the first aspect of the present invention, it is possible to shorten the display switching time.
In this display device, preferably, the colored dispersion medium comprises each of colored dispersion media having a plurality of different colors; the display media are classified into groups each having a large number of the two-color ball type electrophoretic particles and the colored dispersion medium having each of the plurality of different colors; and the one group of the display media forms one pixel. With this configuration, it is possible to realize the display of the colors of the pair of hemispherical portions of each two-color ball type particle and the colors of the plurality of colored dispersion media, and hence to realize the multi-color display.
The two-color ball type electrophoretic particles contained in each of the group of the display media forming the pixel may have an electrophoretic mobility and a rotational threshold voltage which vary from each of the display media. With this configuration, even if a plurality of the display media are driven by applying the same electric field thereto, the action of the two-color ball type particles differs for each of the display media, so that it is possible to realize the display of the color which differs depending on a drive voltage (drive potential) or drive time. For example, by applying an electric field allowing two-color ball type particles having a high drive voltage required for electrophoresis and rotation to sufficiently rotate and migrate, and then applying a reverse electric field allowing only two-color ball type particles having a low drive voltage required for electrophoresis and rotation to rotate and migrate, a state in which only the two-color ball type particles having the high drive voltage required for electrophoresis and rotation have rotated and migrated can be obtained.
The two-color ball type electrophoretic particles contained in each of the group of the display media forming the pixel may have an electrophoretic mobility and a rotational threshold voltage each of which has a distribution in each of the display media. With this configuration, since the visible area of the colored dispersion medium is changed depending on the high or low relationship of the drive voltage or the strong or weak relationship of the electric field required for electrophoresis and rotation, it is possible to realize the display of the color with a different density, that is, color gradation.
Each of the media may be formed by filling a micro-capsule with the dispersion medium and a large number of the two-color ball type electrophoretic particles. With this configuration, like the display device according to the first aspect of the present invention, it is possible to prevent localization of the two-color ball type particles and to simplify the production of the display device.
To achieve the above object, according to a third aspect of the present invention, there is provided a display device including: display media each of which has a dispersion medium and a large number of electrophoretic particles dispersed in the dispersion medium, the electrophoretic particles being driven by applying an electric field to each of the display media, to thereby perform a desired display operation; wherein the electrophoretic particles comprise small-sized two-color ball type electrophoretic particles each of which is composed of a pair of hemispherical portions different from each other in terms of color or reflectance and charging characteristic; the dispersion medium comprises a colored dispersion medium; and one of the pair of hemispherical portions of each of the two-color ball type electrophoretic particles is colored into white and the other is colored into a color having a complementary relationship with a color of the colored dispersion medium.
With this configuration, since the electrophoretic particles are composed of the small-sized dichronic ball type particles each of which is composed of a pair of hemispherical portions different from each other in terms of color or reflectance and charging characteristic, like the first aspect of the present invention, when each two-color ball type particle migrates to the display screen side, the color of one of the pair of hemispherical portions, that is, the color of white or the color having a complementary relationship with the color of the colored dispersion medium is displayed. Accordingly, it is possible to realize the display of clear white. Also, when each of the two-color ball type particle migrates to the opposed side to the display screen side in such a manner that the white hemispherical portion is directed to the opposed side to the display screen side, since light having passed through the colored dispersion medium is absorbed by the hemispherical portion whose color has the complementary relationship with the color of the colored dispersion medium, the portion is appeared black-colored from the display screen side. Accordingly, it is possible to realize the display of true black when viewed from the display screen side as compared with the related art black display which is obtained by making visible mixture of red, green and blue. Further, since the dispersion medium is composed of the colored dispersion medium, when the two-color ball type particle migrates to the opposed side to the display screen side in such a manner that the white-colored hemispherical portion thereof is directed to the display screen side, the color of the colored dispersion medium is displayed. As a result, it is possible to realize the display of a plurality of different colors.
The display device according to the third aspect of the present invention may include a control means for controlling a time required for applying an electric field to each of the display media in such a manner that the electric field is applied to the display medium only for a time required for the two-color ball type electrophoretic particles present on the display screen side of the display medium or the side opposed to the display screen side of the display medium to be reversed, but to migrate before reaching to the opposed side or to the display screen side. With this configuration, like the display device according to the first aspect of the present invention, it is possible to shorten the display switching time.
Further, when an electric field is applied by the above control means in a state in which the two-color ball type particle is present on the display screen side in such a manner that the white hemispherical portion is directed to the display screen side, the two-color ball type particle is reversed in a state being substantially kept at the present position, so that the color having the complementary relationship with the color of the colored dispersion medium is displayed. On the other hand, when an electric field is applied by the above control means in a state in which the two-color ball type particle is present on the opposed side to the display screen in such a manner that the black hemispherical portion is directed to the display screen side, the two-color ball type particle is reversed in a state being substantially kept at the present position, so that the white hemispherical portion is directed to the display screen side, and thereby the color of the colored dispersion medium is displayed. As a result, it is possible to realize the display of four colors by one display medium.
In this display device, preferably, the colored dispersion medium comprises each of colored dispersion media having a plurality of different colors; the display media are classified into groups by color each having a large number of the two-color ball type electrophoretic particles and the colored dispersion medium having each of the plurality of different colors; and the one group of the display media forms one pixel. With this configuration, it is possible to realize the display of the colors of the pair of hemispherical portions of each two-color ball type particle and the colors of the plurality of colored dispersion media, and hence to realize the multi-color display.
The two-color ball type electrophoretic particles contained in each of the group of the display media forming the pixel may have an electrophoretic mobility and a rotational threshold voltage which vary from each of the display media. With this configuration, even if a plurality of the display media are driven by applying the same electric field thereto, the action of the two-color ball type particles differs for each of the display media, so that it is possible to realize the display of the color which differs depending on a drive voltage (drive potential) or drive time. For example, by applying an electric field allowing two-color ball type particles having a high drive voltage required for electrophoresis and rotation to sufficiently rotate and migrate, and then applying a reverse electric field allowing two-color ball type particles having a low drive voltage required for electrophoresis and rotation to rotate and migrate, a state in which only the two-color ball type particles having the high drive voltage required for electrophoresis and rotation have rotated and migrated can be obtained.
The two-color ball type electrophoretic particles contained in each of the group of the display media forming the pixel may have an electrophoretic mobility and a rotational threshold voltage each of which has a distribution in each of the display media. With this configuration, like the display device according to second aspect of the present invention, since the visible area of the dispersion medium is changed depending on the high or low relationship of the drive voltage or the strong or weak relationship of the electric field required for electrophoresis and rotation, it is possible to realize the display of the color with a different density, that is, color gradation.
Each of the media may be formed by filling a micro-capsule with the dispersion medium and the two-color ball type electrophoretic particles. With this configuration, like the display device according to the first aspect of the present invention, it is possible to prevent localization of the two-color ball type particles and to simplify the production of the display device.
To achieve the above object, according to a fourth aspect of the present invention, there is provided a method of driving a display device including the steps of: preparing a display device including pixels each of which has a plurality of display media, each of the display media being formed by dispersing a large number of small-sized two-color ball type electrophoretic particles, each being composed of a pair of hemispherical portions different from each other in terms of color or reflectance and charging characteristic, in each of colored dispersion media having a plurality of colors, wherein a drive voltage of the two-color ball type particles differs for each of the display media; and applying an electric field to each of the display media, to drive the two-color ball type electrophoretic particles, thereby performing a desired display operation; wherein the two-color ball type electrophoretic particles in each of the plurality of display media are classified into a first group having a high drive voltage and a second group having a low drive voltage; an electric field allowing the first group having a high drive voltage to sufficiently rotate and migrate is applied and then a reverse electric field allowing only the second group having a low drive voltage to rotate and migrate is applied, to form a state in which only the first group having a high drive voltage has rotated and migrated.
With this configuration, since an electric field allowing the first group having a high drive voltage to sufficiently rotate and migrate is applied and then a reverse electric field allowing only the second group having a low drive voltage to rotate and migrate is applied, to form a state in which only the two-color ball type particles of the first group having a high drive voltage has rotated and migrated, the visible area of the colored dispersion medium is changed depending on the high or low relationship of the drive voltage or the strong or weak relationship of the electric field. As a result, it is possible to realize the display of different colors and the display of the color with a different density (gradation display).